The present invention relates to an information recording and reproducing apparatus including signal conversion means for recording or reproducing information from a disc-shaped recording medium including tracks on which information has been recorded or a spiral track on which information is to be recorded.
High-density recording techniques have recently been known in the art in which a disc is formed by using a light-sensitive recording material and, while rotating the disc, a beam of light such as laser is converged into a tiny spot of less than 1 .mu.m in diameter and then projected onto the disc thereby accomplishing high-density recording of signals in the form of variations, e.g., white/dark variations, hole or pit variations or intensity variations on the disc or reproducing such signals.
For instance, included among these known techniques are optical video discs which are well known as apparatus for reproducing the signals recorded at high density on the discs.
Also, the above-mentioned signal recording technique is utilized in the production of a master for video discs.
As regards the form of signals to be recorded, the recording of video signals, sound signals, digital signals and the like has been conceived.
In the above-mentioned optical information recording and reproducing apparatus, the recording of a signal is effected by projecting a laser beam onto the recording thin film on the disc so that the area of the thin film exposed to the laser beam is fused and evaporated or the reflectance or transmittance of the thin film is changed. In other words, it is the usual practice to change the optical characteristic of the recording material by thermally utilizing the energy of the laser beam.
To accomplish high track-density recording of the optical recording disc, the disc is preliminarily formed with guide tracks of a groove structure and signals are recorded while performing a tracking control on the guide tracks. The guide tracks have the effect of reducing the positional deviation of the recording laser beam due to vibrations of the apparatus and allowing recording of signals in any selected locations of the optical recording disc.
Also, by recording a specific address to each of the tracks during the master cutting of the optical recording disc, it is possible to randomly search any given one of all the tracks on the optical recording disc.
The guide tracks are selected in accordance with the contents of information or signals to be recorded and generally the tracks are formed in the spiral or concentric form with respect to the center of the disc.
As regards the track form, the spiral track is more advantageous than the concentric tracks from the track density point of view. The reason is that the track feed accuracy of the master cutting machine is such that the occurrence of vibrations due to the track feed is eliminated and the effect of backlash of the feed screw mechanism is reduced in the case of the spiral track requiring the continuous feed than in the case of the concentric tracks requiring the intermittent feed.
However, in the case of the optical recording disc employing the spiral track, differing from the concentric tracks, the track is a single continuous one on the optical recording disc and therefore it is difficult to hold the optical head on the track of a desired address as compared with the concentric tracks. With the spiral track, in order to perform a tracking control concentrically on any given track of the same address, it is necessary to force the beam spot from the optical head to skip or jump by one track. In this case, in response to the detection of a rotational position signal (hereinafter referred to as an index signal) preliminarily provided on the track in the tracking mode, a jumping signal of a predetermined amplitude is externally applied to the optical head and it is shifted in the radial direction of the optical recording disc. Thus, when the optical head moves away from the current track and comes onto the next guide track, braking is applied to the optical head and thus it performs a still picture operation of reproducing again the previously reproduced track.
With the above-described construction, however, when a flaw is caused or foreign particles are deposited on the index portion preliminarily formed on the track, a dropout occurs in the reproduced signals thus failing to detect the index signal. When this occurs, the optical head cannot perform the still picture operation since it reproduces the next track due to the track being spiral in form.